I am always puzzled by the condition that creates these hot spots. I have no proper technical knowledge of what causes the but I have tried to explain them on forums to the best of my understanding. (For want of an alternative more knowledgeable source). It seems that the "spots" are shown and commented upon but their technical reason for occurring remains a mystery surronded in optical jargon.

If I repeat my understanding here then you might be able to comment and confirm or advise differently.

My understanding is that back in the film days the film stock was not as reflective as modern sensors and therefore the reflection of a strong light source from the film and further re-relection back off some part of the lens glass back on to the film was less of a problem. In fact some lens manufacturers did not particularly worry about it as inconsequential. Different legacy slr lenses are therefore more prone to sensor re-reflection spots/smudges than others.

Sensor reflection can be bounced back via some flatter glass surface in the form of a aperture delineated circle if from an element located in front of the aperture or as a smudge if after the aperture. According to some Canon literature even a flat filter lens surface can cause this reflection. Canon seem quite proud that they have a meniscus protective front lens to prevent this re-reflection "unlike some of their opposition". Canon's advice for those getting this hot spot on their esteemed telephoto range in extreme conditions was to try removing the lens filter.

I believe/think that good lens design can disperse this re-reflection before it is completely reflected back to the sensor surface. You are to be congratulated as the Metabones product seems to have been designed to reduce or eliminate this re-reflection that happens in more extreme circumstances very well.

I also have to make a disclaimer so that readers might understand: I share your surname, but we are not related. I have no commercial connection with Metabones nor any other Chinese focal reducer manufacturer. I buy my own focal reducers, including a Metabones Speed Booster and several other Chinese made focal reducers for the Sony E-mount. I have one RJ made M42 to M4/3 focal reducer adapter. I am not suitably qualified to make exacting tests, nor do I have any optical knowledge other than what I have read.

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Tom Caldwell

Hi Tom:

Central hotspots are a ghost image of the aperture stop. The visibility of the ghost image depends on the magnification of the ghost image (smaller means brighter), the amount of aberration in the ghost image (less aberration means a more pronounced ghost), and the efficiency of the lens coatings.